This invention relates generally to endoluminal grafts or xe2x80x9cstentsxe2x80x9d and, more specifically, to stent delivery systems and methods.
A stent is an elongated device used to support an intraluminal wall. In the case of a stenosis, a stent provides an unobstructed conduit through a body lumen in the area of the stenosis. Such a stent may also have a prosthetic graft layer of fabric or covering lining the inside and/or outside thereof. Such a covered stent is commonly referred to in the art as an intraluminal prosthesis, an endoluminal or endovascular graft (EVG), or a stent-graft. As used herein, however, the term xe2x80x9cstentxe2x80x9d is a shorthand reference referring to a covered or uncovered such device.
A covered stent may be used, for example, to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, a stent is implanted in a blood vessel at the site of a stenosis or aneurysm endoluminally, i.e. by so-called xe2x80x9cminimally invasive techniquesxe2x80x9d in which the stent, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a stent delivery system or xe2x80x9cintroducerxe2x80x9d to the site where it is required. The introducer may enter the body from an access location outside the body, such as through the patient""s skin, or by a xe2x80x9ccut downxe2x80x9d technique in which the entry blood vessel is exposed by minor surgical means. The term xe2x80x9cproximalxe2x80x9d as used herein refers to portions of the stent or delivery system relatively closer to this access location, whereas the term xe2x80x9cdistalxe2x80x9d is used to refer to portions farther from the access location.
When the introducer has been threaded into the body lumen to the stent deployment location, the introducer is manipulated to cause the stent to be ejected from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the stent), whereupon the stent expands to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stent expansion may be effected by spring elasticity, balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration.
For a self-expanding stent, the delivery system typically comprises at least an outer sheath, the compressed stent within the outer sheath, and a stabilizer proximal of the compressed stent. The delivery system may further comprise an inner member that runs through the compressed stent and that attaches to a catheter tip distal of the stent. The term xe2x80x9cstabilizerxe2x80x9d is used in the art to describe a component of stent delivery systems used to stabilize or prevent retraction of the stent when the sheath is retracted. The stabilizer thus effects deployment of the stent into a desired location by forcing relative movement between the sheath and the stent. A stabilizer handle is typically located at the proximal end of stabilizer, outside the body lumen. To deploy the stent, the delivery system is threaded through the body lumen to a desired location for stent deployment. The outer sheath is then retracted, and the stabilizer prevents the stent from retracting proximally along with the sheath. As soon as the outer sheath constraining the stent is retracted, the stent typically expands into place.
For a balloon-expandable stent, the stent delivery system typically comprises the compressed stent preloaded around an inflatable balloon. The stent and underlying balloon are navigated to the desired deployment location and then the balloon is inflated to expand the stent into place.
In other embodiments, balloon-expandable or self-expanding stents may be used with a guide catheter which is first navigated through the lumen, such as over a guide wire, until its distal end is adjacent a deployment location. The guide catheter is then used as an outer sheath through which the stent is threaded. In such configurations, balloon-expandable stents have been known to come uncoupled from their underlying balloon inside the guide catheter, requiring that the entire catheter be removed and discarded.
With both self-expanding and balloon-expandable stents, the expansion process may cause unwanted movement of the stent. For example, with a balloon expandable stent, the proximal end of the balloon may inflate first. This may be due to, among other things, the location of the inflation lumen orifice (which may be at the proximal end of the balloon), variations in the stent geometry or in the crimping force used to compress the stent, variations in the folds of the balloon, and/or compressible air bubbles due to an incomplete balloon air purge prior to inflation with a non-compressible fluid. The result may be momentary instability in the stent position on the balloon, where axial force from the inflating balloon may overcome static friction between the balloon surface and the stent, resulting in axial migration of the stent. The axial migration may result in inaccurate deployment or outright embolization of the partially deployed stent.
Stents may also be shorter in their expanded configuration as compared to their compressed configuration. Thus, the expansion process may also cause the stent to shorten as its diameter expands. This shortening may also undesirably move the stent relative to its desired deployment position. The accurate placement of self-expanding stents may also be jeopardized during the deployment process.
Accurate placement of stents, especially short stents, such as used to repair ostial lesions, is important. For example, in ostial lesions of the renal arteries, stent misplacement can result in complications including a necessity for additional stent placement, stent migration and retrieval from the distal aorta, and renal failure. Given the importance of accurate stent placement in certain applications, an improvement to currently-available stents and stent delivery systems is desired.
In accordance with this invention, there is provided a stent delivery system comprising a stent and at least one tether detachably connected to the stent by a connecting member. The tether may connect to the stent in a proximal, distal, or medial location, and the stent may be balloon-expandable or self-expanding.
In one embodiment, the tether may comprise a conductive material and the connecting member may comprise a material subject to degradation upon application of an electrical current, such as a material that corrodes preferentially relative to the tether material. For example, the tether may comprise an insulated wire and the connecting member may comprise an uninsulated portion of the tether wire. In another embodiment, the connecting member may comprise a solder, a brazing paste, or an adhesive that has an impedance higher than the impedance of the tether.
In another embodiment, the connecting member, the stent, and the tether each have a mechanical strength, where the strength of the connecting member is less than the strength of the stent and the tether. The connecting member may have a first mechanical strength prior to application of electrical current and a second mechanical strength after application of an electrical current of sufficient magnitude and duration to degrade the first mechanical strength such that the second mechanical strength is less than both the mechanical strength of the tether and the mechanical strength of the stent. The connecting member may comprise a cross-sectional area that is smaller than the cross-sectional sectional areas of the tether and the stent.
The invention also comprises a method of deploying a stent using the stent delivery device comprising a tether attached to the stent by a connecting member. The method comprises the steps of inserting the stent delivery device into a lumen through an access location; navigating the stent delivery device to a deployment location; expanding the stent from a compressed configuration to an expanded configuration while using the tether to maintain the stent in the deployment location; and detaching the tether from the stent. The method may comprise repositioning or reconstraining the stent using the tether once the stent is in an expanded or partially-expanded configuration. Detaching the tether may comprise applying an electrical current to the tether that causes the connecting member to degrade, applying a tensional or torsional force to the tether that causes the connecting member to detach, or a combination thereof.